WhatsApp now has 400 million monthly active users, or people who use the app at least once in a month, according to a Dec. 19 company announcement.

The app added an astounding 100 million monthly active users (MAUs) in the four months leading up to the announcement.

Facebook, Twitter, and other platforms really need to watch out, or they may see a huge part of their user bases spend more and more time on WhatsApp — which offers a far less cluttered and ad-free environment.

WhatsApp's closest competitors are from Asia: China-based WeChat and Japan-based LINE, which we estimate ended November at 297 million and 186 million MAUs, respectively.

LINE is also big in some non-Asian countries like Spain. It's pushing aggressively to take on WhatsApp in more markets. But so far WhatsApp looks to be way out of reach.

For additional context on how big these apps are, consider that the headline-grabbing photo-messaging app Snapchat has just 30 million MAUs, according to reporting by Business Insider's Alyson Shontell.

On the Android side, WhatsApp is clearly dominant BI Intelligence Facebook has its own messaging app, called Facebook Messenger, but it had just 53 million MAUs earlier this year, according to tech analyst Benedict Evans.

For further confirmation of WhatsApp's new dominance, check out this November 2013 data on Android users from wireless data company Mobidia:

WhatsApp is on 41% of Android smartphones, while another Asian messaging app, Kakao Talk, is on 23% of Android phones. Mobidia gets these numbers from its free apps that people download so that they can keep tabs on their data usage. The apps are on millions of phones globally.

Kakao Talk is big in South Korea, where Android totally dominates the smartphone market. That helps explain why Kakao Talk looks so big in this data.

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